1. Field of the Invention
The present invention relates to methods and apparatus for improving the performance of multimode optical fibre communication systems, and in particular to methods and apparatus for use in connecting optical transceivers to multimode fibres.
2. History of the Prior Art
In the late 1970s, and early 1980s, much work was carried out to improve performance of multimode optical fibre communications systems. However, when multimode fibre was replaced by a singlemode fibre, as the medium of choice for use in high bit rate, long distance communications systems, much of this work ceased. Multimode fibre has continued to be used in optical communications for systems operating at lower bit rates, and over shorter distances, for example in building or campus LANs. Such multimode fibres are predominantly used in the LAN backbone but may also be used in horizontal links to users and appliances. There is thus a large installed base of multimode fibre, which represents a significant investment.
In recent years the demand for high data rate LANs has increased dramatically, for example to 1 GBit/s and beyond. The required data rates cannot be achieved utilising conventional techniques with LANs containing significant multimode fibre, even when the lengths of fibre are relatively short (500 meters).
A key aspect in determining the bandwidth of a multimode optical fibre communications link, which has been recognised for many years, is the number and distribution of modes within the multimode fibre which are excited, and therefore carry optical energy. See for example Chapter 7 of "Optical Fibres for Transmission" by John E. Midwinter, published by John Wiley & Sons in 1979. If a pure low order single mode is launched into a multimode fibre, and there is no mode mixing, the bandwidth and other characteristics of the optical communication link will be that of a single-mode fibre, i.e. the link will have high bandwidth. If mode mixing occurs, for example due to fibre profile irregularities, or mechanical perturbations of the fibre, energy will be coupled from the single lowest order mode into higher order modes having higher group velocities, and additional pulse dispersion will inevitably result, leading to a lower overall bandwidth for the communications system. Alternatively, if light is launched into the same multimode fibre in a manner so as to uniformally excite all modes of the multimode fibre, and if no mode mixing occurs, a maximum pulse spread will be seen, and the bandwidth of the communications system will be at a minimum. If mode mixing is introduced to this situation, because individual photons will then spend some time in many different modes, and will have travelled many short distances at different group velocities, less pulse spreading will be experienced. In the ideal case rather than experiencing an increase of pulse spreading which is proportional to the length of the optical communications link, pulse spreading builds up only in proportion of the square root of the length of the optical communications link. Thus, in the early 1980s, although various alternative schemes were investigated (see eg U.S. Pat. No. 4,050,782 and U.S. Pat. No. 4,067,642), it was generally accepted that it was desirable to launch many modes into a multimode optical fibre, and to ensure that adequate mode mixing occurred in order to achieve a reasonable, and predictable, bandwidth for an optical communications link.
Despite this practical approach, it was however theoretically predicted that if the number and distribution of modes excited within a multimode fibre could be precisely controlled, the bandwidth of the communications link could be improved. For example, see Section 7.6, page 126 of Midwinter's book where it is suggested that controlled mode coupling can be utilised to prevent coupling to the highest order modes of the fiber so as to increase the fibre bandwidth without incurring a loss penalty. Nevertheless, it is stated here that "It must be said, however that experimentally it looks extremely difficult to achieve such a precisely controlled fibre environment, and at the time of writing no reports of experimental testing are known."
In recent years lasers rather than LEDs (Light Emitted Diodes) have been utilised with multimode optical fibre communications systems. There are a number of reasons for this, of which the predominant one is that lasers can be directly modulated at higher speeds than LEDs. In contrast to LEDs, lasers can easily be utilised to excite only a few, low order modes of the multimode optical fibre. As discussed above, if only a few modes of a multimode fibre are excited, and little mode mixing occurs, the bandwidth of a multimode optical fibre communications system can in principle be increased somewhat. For example, data rates up to 1 GBit/s have been achieved over a maximum of 200 meters using a 780 .mu.m laser diode and 62.5 .mu.m multimode fibre.
There are significant differences between use of an LED and use of a laser in launch of a signal into multimode fibre. Characteristically, an LED launch will be an overfilled launch and hence will cause the modes of the multimode fibre to be fully populated. Bandwidth of multimode fibre is characterised according to its performance for such a launch. However, as indicated above, a laser does not have an overfilled launch--instead, there will be a restricted launch in which only certain of the fibre modes will be partially populated or largely unpopulated. The nature of the restriction of the launch is dependent on a number of factors--lower numerical aperture of the laser than the multimode fibre, smaller spot size than core diameter, nature of the laser source and coupling arrangement (constituents of the coupling mechanism such as lenses, fibre stubs etc.).
The present inventors have found that restricted launch into a multimode fibre can have a serious effect on the bandwidth achieveable with the fibre, even where the bandwidth of the fibre is nominally in specification according to the overfilled launch bandwidth. The particular difficulty found is that the bandwidth exhibited by a fibre is strongly dependent on the details of the restricted launch. The actual bandwidth achieved can be significantly higher than the overfilled launch bandwidth--it can also be significantly lower. This creates a serious problem for system designers, as it is thus not possible to guarantee what minimum bandwidth will be encountered.